In heating boilers, particularly in heating boilers with a burner for liquid or gaseous fuels, it is usual to conduct the combustion gases emerging from a combustion chamber through fire tubes before they leave the heating boiler. The fire tubes traverse the water compartment of the boiler which surrounds the combustion chamber, and form ancillary heating surfaces for the better utilization of the heat of the combustion gases. Cylindrical fire tubes are known which are provided with inwardly stamped ribs or currugations in the cylindrical tube wall in order to increase the turbulence of the combustion gases in the fire tubes and the heat transfer from the combustion gases to the fire tube. This means only leads to a satisfactory result in tubes with a comparatively small cross-section. However, small tube cross-sections necessitate a large number of individual flue-gas tubes, and therefore a large number of watertight welds of the tube ends into the boiler walls.
Also known are fire tubes with a square cross-section, for example, which consist of two tube halves of metal, shaped at an angle, which are welded together at their edges abutting at two opposite corners of the square cross-section. At the abutting edges, one of the two tube halves is provided with an angled bend directed towards the inside of the tube. These fire tubes therefore have only in the two opposite corners of the square cross-section, where the tube halves are welded together, a metal strip projecting diagonally into the interior of the tube as an additional heating surface of the fire tube.
In the present invention two U-shaped tube halves are constructed with a broad profile base and with short profile arms bent at an angle so that the two tube halves form in cross-section a pocket-like flat tube with two broad plane side surfaces and two narrow side surfaces on which longitudinal welding seams connect the profile arms of the two tube halves. These pocket-like flat fire tubes have a comparatively large cross-section so that the ancillary heating surface of a heating boiler can be formed with a smaller number of such fire tubes. With these pocket-like flat fire tubes, it is known to provide the broad plane side surfaces of the tube with corrugations extending transversely to the longitudinal direction of the tube and projecting inwards so as to achieve, in this manner, an increase in the heat transfer from the combustion gases to the ancillary heating surface formed by the fire tube. In comparison, the construction of such a pocket-like flat fire tube according to the present invention is such as to have the advantage that a significant increase in the heating surface and the heat-exchange performance of the fire tube is achieved with simple means which are inexpensive to produce. Consequently, the burner capacity of a heating boiler can be increased without an unwanted increase in the waste-gas temperature of the boiler, or it is possible to manage with an even smaller number of fire tubes as the ancillary heating surface of a heating boiler with a predetermined burner capacity and waste-gas temperature.
According to one embodiment of the invention L-shaped metal strips arranged side-by-side like a comb on the inner profile base of a tube half may be separate metal strips which can be welded in pairs to the profile base very easily and simply by a single common welding seam, for example with automatic welding machines. Also it is a particular advantage to form two such L-shaped metal strips integrally from a single metal strip which is bent into a U-shape configuration and which, as a result of its U-shaped configuration, is provided with a stamped out longitudinal slot which is interrupted by narrow transverse webs holding together the two L-shaped halves of the U-section. The U-section can be welded in a simple manner, for example by welding with an automatic machine, to the profile base of the tube half, firmly and with good heat conduction, through the longitudinal slot.
Other features which are considered characteristic of the invention are set forth in the appended claims.
Although the invention is illustrated and described in relationship to specific embodiments, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.